A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. Efforts are made to design turbine blades so that they do not resonate at the normal operating speed and critical harmonics of the speed of the machine. Otherwise, blade resonances excited by rotational speed may create stresses which break the blade and cause extensive damage, thus shutting the machine down and requiring extensive repair. In order to avoid the aforementioned problem, detailed testing is performed prior to operation of a machine to ensure that blades will not resonate during normal operation.
It is also desirable to monitor rotating blades during operation in order to identify vibration problems which develop after a turbo machine is put in use. This on-line evaluation is necessary in part because evaluations performed prior to actual use do not subject the blades to the same temperature, pressure and rotational conditions which are experienced during normal operations. Continuous monitoring of blade vibrations is also important in order to detect new vibrations which signal structural changes. If any of these vibrations escape detection, developing fractures will likely lead to extensive damage and costly down time while the machine undergoes repair.
Although previous methods of performing off-line evaluations have successfully eliminated many serious vibration problems, system designs for on-line detection have not provided for the reliable and comprehensive monitoring which is desired in order to avoid the above described problems. Systems having a limited capability for monitoring on-line blade vibrations have utilized a plurality of permanently installed non-contacting sensors. These sensors are radially disposed about the rotating blades in order to monitor vibration of individual blade tips about their normal positions in a rotating time frame. An exemplary design is disclosed in U.S. Pat. No. 4,573,358 to Luongo. This and other systems designed to provide vibration signals to analysis equipment have several limitations affecting their suitability for monitoring turbo machinery at steady or synchronous speeds without interruption of operation. None of these systems are believed suitable for continuously monitoring at steady or synchronous speeds the multitude of blades found in turbo machinery in order to quickly detect new structural defects and take a machine off-line before extensive damage occurs. For example, the generally recognized phenomenon of misalignment, a static effect found in permanently mounted sensor systems, results in distortion of the vibration wave forms which are reconstructed from blade displacement data. Prior efforts to introduce compensation factors for these shifts have been essentially one time corrections which are not suitable for monitoring machinery during steady or synchronous speed operation. The technique disclosed by Luongo introduces corrections for sensor drift based on low speed rotational data taken between known resonant frequencies of the blades in order to monitor blade position at frequencies for which vibrations are believed to be absent. While collection of correction data at frequencies below normal operating speeds does provide for an accurate determination of sensor position in the absence of any blade resonances, it is often not possible or convenient to vary the speed of an operating machine in order to acquire this data. This constraint is present in power plants which must keep turbines running at synchronous speed in order to maintain constant electrical power frequency. Furthermore, correction data acquired at lower than operating speeds cannot take into consideration the differing dynamic effects between measurement speeds and operating speeds. For example, thermal expansion and centripetal forces further alter sensor alignments once the machine resumes operational speed. Nor does application of this approach to on-line machines lend itself to continuous correction of significant errors which are due to sensor displacement. Absent an improved static and dynamic correction technique for sensor misalignment, data acquired through continuous monitoring tends to become distorted as sensor positions drift. Therefore, it is believed that a continuous correction technique is necessary to further minimize sensor drift effects which lead to undesirable noise in the signal analysis. Such an improved correction technique should allow for effective monitoring of blade tip vibration without any interruption of equipment operations.